It's a bit confusing that it has to be enabled seperately from the SSL tab, but also makes sense somehow
The reason for separating the settings is that ispconfig is made for hosting companys. There you have administrators and client. The ssl checkbox exists so that a administrator can enable / disable the ability for the client to have a ssl cert and all these website limits are on the first tab. So when its active, the client can create and manage its ssl cert on the ssl tab.